This invention relates to appliance timers and particularly to a drive mechanism for the timing cam of an appliance timer.
Although the principles of the present invention may be included in various timers, a particular application is made in timers of the type employed to control the various circuits and functions of automatic laundry appliances, such as washers and dryers. Appliance timers basically comprise a rotatable member having a plurality of projecting cams which sequentially engage switch members which control the various operations of the appliance.
Basically, there are two types of timers for home appliances. One such timer is a cylindrical rotary drum having a plurality of cams projecting from its surface which engage switch members and is quite similar to a music box. A principal disadvantage of such a timer is that it is bulky and it is difficult to mount the timer in the limited space requirements dictated by the appliance manufacturer. A more acceptable timer as far as space requirements are concerned is the timer set forth in U.S. Pat. No. 2,703,347 to A. R. Constantine, issued Mar. 1, 1955. According to that patent, the appliance timer comprises a flat disc mounted in a housing and having ratchet teeth on its periphery. The disc is driven incrementally about a central shaft by a pawl, which in turn is driven in a reciprocal manner by a synchronous motor. At least one face of the disc is provided with arcuate cam surfaces which are concentrically arranged about the face or faces of the disc. Switch members are arranged to engage the arcuate cam members upon rotation of the disc, and control various cycles of the appliance.
While the teachings of the present invention are set forth herein with particular reference to a disc cam, it is to be understood that the invention may also be employed as a drive mechanism for a drum cam. In either case, it is desirable that the disc or drum be driven at a very slow rate of advance, since one rotation of the disc or drum should complete at least one entire cycle of the machine. Therefore, it is desirable to decrease the angular spacing between the ratchet teeth, and therefore shorten the stroke of the pawl to that spacing, and/or decrease the pawl stroke frequency by gear reduction from the drive motor.
Reducing the tooth-to-tooth spacing necessarily reduces the size of the teeth, and increases the possibility of the pawl to misengage or slip the ratchet teeth. Moreover, reducing the length of stroke of the pawl presents problems in designing suitable cam drives between the motor and the pawl. Generally, the cam disc is designed so that tooth-to-tooth spacing on a disc having a diameter of about 41/4 inches is between 21/2 to 8 degrees.
In addition to the problems presented in the design of mechanisms for precise pawl and ratchet cooperation, manufacturing tolerances for those mechanisms must be closely controlled to minimize malfunctions.